The present invention relates to a pain sensory nerve stimulation apparatus which, with respect to the primary pain sense produced by stimulation of Aδ fibers, and the secondary pain sense produced by stimulation of C fibers, can perform only stimulation of C fiber.
In order to stimulate only the pain sense by electrical stimulation, an electrode disclosed in Patent Reference 1 has been developed. According to the electrode, it is possible to stimulate Aδ fibers (see FIG. 3 of Patent Reference 1).
By contrast, in order to early detect a disorder of peripheral nerve which is one of the three major complications of diabetes, a method in which only C fibers are stimulated and a reaction to the stimulation is checked is highly requested. This is based on that C fibers have a thickness of 0.4 to 1.2 μm, Aδ fibers have a thickness of 2 to 5 μm, Aβ fibers which are connected to mechanoreceptors of the tactile sense, the pressure sense, and the like have a thickness of 5 to 12 μm, and a nervous disorder begins from small fibers. When it possible to stimulate C fibers which are smaller than Aδ fibers that can be stimulated in Patent Reference 1, therefore, development of a nervous disorder can be known more early, thereby largely contributing to knowing of progression of and adequate control of a diabetic nervous disorder. In a related-art technique of stimulating only C fibers, a laser apparatus is used. In the case where a laser apparatus is used, however, large-scale testing equipment and facility are required, and the size, cost, and complexity of the apparatus are increased to cause a problem of versatility. The accuracy of C fiber stimulation by laser light has not yet reached to a satisfactory level, and the probability of C fiber stimulation is lower. Therefore, a laser apparatus has not been used in a clinical application.
Also in Patent Reference 1, when stimulation is applied while a needle terminal of the stimulation electrode is set as a negative pole and the surrounding electrode is set as a positive pole, it is possible to selectively stimulate A6 fibers, but C fibers cannot be selectively stimulated.
In the field of the art, a related-art technique in which electrical stimulation of the negative polarity is applied to a stimulation object electrode attached to a desired stimulation portion, and the positive polarity is applied to an end electrode is employed because excitation of peripheral nerves is generated immediately below a stimulation electrode of the negative polarity. Also Patent Reference 1 describes that stimulation is applied while the needle electrode of the stimulation electrode is set as a negative pole and the surrounding electrode is set as a positive pole, so that Aδ fibers can be selectively stimulated. However, in the field, even when the electrical polarity is inverted, the inversion exerts no effect or influence on nerve stimulation from the skin surface which is used in a measurement of the somatosensory evoked potential SEP (see FIG. 9).
Patent Reference 2 discloses a related-art apparatus which automatically measures in an electrophysiologic or quantitative manner the electrical current perceptive threshold and the algestic tolerant threshold. In the to related-art apparatus, stimulation is applied by using a sine wave, and C fibers, Aδ fibers, and Aβ fibers are most responsive to stimulation of frequencies of 5 Hz, 250 Hz, and 2,000 Hz, respectively. Patent Reference 2 does not provide a technique in which C fibers, Aδ fibers, and Aβ fibers can be independently stimulated. In the related-art apparatus, stimulation due to a sine wave is required. Therefore, the apparatus is complicated in structure for producing and controlling stimulation, as compared with the case where stimulation is applied by a pulse wave which is typified by a square wave.
The technique disclosed in Patent Reference 2 uses a surface electrode, and hence the stimulation intensity to be applied is larger than the case where an electrode is inserted into the skin. In stimulation of small fibers such as C fibers, namely, there is a large possibility that also other nerve fibers such as the tactile sense are stimulated. Therefore, it is considered that selective stimulation of only C fibers is difficult.
Patent Reference 3 discloses a related-art technique in which specific tactile sensory receptors are selectively stimulated by using a surface electrode and without performing stimulation on the pain sense. This technique is effective in stimulating the tactile sense. Since the surface electrode is used, however, the stimulation intensity is as large as about 2 mA. Furthermore, Aδ fibers and C fibers which relate to the pain sense are small fibers, and hence stimulation is hardly performed. In the related-art technique disclosed in Patent Reference 3, therefore, it is impossible to selectively stimulate Aδ fibers and C fibers. The related-art technique disclosed in Patent Reference 3 has a further problem in that a complex process such as a weighted change is required in selective stimulation of receptors.
In electrical stimulation, excitation is more easily caused in the sequence of Aβ fibers, Aδ fibers, and C fibers. Namely, electrical stimulation is more easily performed on larger medullated fibers having an axial fiber, and excitation due to electrical stimulation most hardly occurs in C fibers which are small fibers, and which are nonmedullated fibers. Therefore, stimulation in which only C fibers are selectively stimulated without stimulating other fibers is very difficult.
[Patent Reference 1] JP-T-2006-59430
[Patent Reference 2] U.S. Pat. No. 5,806,522
[Patent Reference 3] Japanese Patent No. 3,543,097